1. Field of the Invention
The present invention relates to an apparatus for forming an image, such as an electrophotographic duplicator and a facsimile machine, and in particular, it relates to an improvement of an apparatus for forming an image, in which rotation of rotational members including an electrostatic latent image carrying member and other rotational members is borne by an electroconductive rolling bearing.
2. Description of Related Art
An apparatus for forming an image, such as an electrophotographic duplicator and a laser beam printer, contains various kinds of rotational members, such as a photoreceptor drum, a transferring roll, a magnet roll of a developing device, a heating roll of a fixing device and a transporting roll for a recording sheet or an original copy, and the rotational members are borne by a fixed member, such as a frame of the apparatus, through rolling bearings. In an electrophotographic process, a photoreceptor drum is uniformly charged, and then the charge is relieved only in the exposed part to form an electrostatic latent image. Therefore, the photoreceptor drum is necessarily grounded. In order to develop the electrostatic latent image formed on the photoreceptor drum, a developing bias is necessarily applied to the magnetic roll, and in order to transfer a toner image from the photoreceptor drum to the recording sheet, a transferring bias is necessarily applied to the transferring roll. Furthermore, the heating roll and the transporting roll for a recording sheet are sometimes charged due to the charge carried by the recording sheet, and when a discharge phenomenon due to the charge occurs, malfunction and unnecessary noise caused by radiated electric waves occur. In order to prevent the problems, the rolls are necessarily grounded. As a method for grounding the various kinds of rotational members or connecting them to a bias electric power source, such a method has been know that a spring is made in contact under sliding with one end or both ends of the rotational member, whereby connection and grounding of the various kinds of rotational members are attained.
However, because the slide-contact between the spring and the rotational member brings about time-lapse deterioration, such as wear and alteration, fluctuation of the resistance between the spring and the rotational member is unavoidable, and therefore not only the initial performance cannot be maintained with the lapse of time, but also there is a possibility that grounding or connecting failure and abnormal noise occur.
An electroconductive bearing has been known that assures electric conduction between an inner ring and an outer ring without provision of any particular slide-contacting member. When the rotation of the rotational member is borne by using the electroconductive bearing, the rotational member can be grounded or applied with a high bias voltage without providing a spring. The electroconductive bearing contains an outer ring fixed to a fixed member, such as a frame of an apparatus, an inner ring fixed to a rotational axis, and rolling elements, such as balls and rollers, rolling between the outer ring and the inner ring under receiving a load, in which the inner and outer rings and the rolling elements are formed with an electroconductive metal, such as steel. Grease filled between the inner ring and the outer ring is also imparted with electroconductivity, and even when an oil film is formed between the rolling elements and the inner ring or between the rolling elements and the outer ring, electric conduction can be assured.
However, in the case where a rotational member, such as a photoreceptor drum, is grounded by using the conventional electroconductive bearing, another problem arises that unallowable radiation noise occurs at the electroconductive bearing, although the function of grounding the rotational member is exerted without any problem. An electronic office equipment, such as an electrophotographic duplicator and a facsimile machine, sometimes generate jamming (radiation noise) over wide frequency bands to cause disturbance in receivers, such as radios and televisions. In order to prevent the electric wave disturbance, a self-imposed standard has been determined by the manufacturer of the equipment in Japan. However, in the case where the conventional electroconductive bearing is used, the radiation noise generated by the bearing cannot be controlled within the range of the self-imposed standard. As a reason of the generation of radiation noise, it is considered that a slight gap is formed between the inner ring or the outer ring and the rolling elements during rotation of the rotational member, and electric leakage occurs at the gap.
The invention has been made in view of the problems associated with the conventional art and provides an apparatus for forming an image, in which a rotational member, such as a photoreceptor drum, is borne by using a rolling bearing assuring electric conduction between an inner ring and an outer ring by filling electroconductive grease, and generation of radiation noise from a rotational part using the bearing due to leakage can be effectively suppressed even when the rotational member is grounded without any other slide-contact member, such as an grounding spring.
The invention relates to, as one aspect, an apparatus for forming an image containing an electrostatic latent image carrying member and an electroconductive rolling bearing that bears rotation of the electrostatic latent image carrying member; the electroconductive rolling bearing containing an outer ring and an inner ring, which are fixed to a pair of members relatively rotating, respectively, plural rolling elements rolling between the inner ring and the outer ring under receiving a load, and a lubricant filled between the outer ring and the inner ring that contains an electroconductive substance dispersed therein; a resistance between the inner ring and the outer ring being about 15 kxcexa9 or less in terms of a maximum value and about 8 kxcexa9 or less in terms of an effective value under a dynamic condition, where a radial or thrust load acting between the inner ring and the outer ring is 10 N, and a relative difference in rotational velocity between the inner ring and the outer ring is 150 rpm; and the electroconductive rolling bearing being used at a relative difference in rotational velocity between the inner ring and the outer ring of about 75 rpm or more.
It is preferred that the resistance between the inner ring and the outer ring is 11 kxcexa9 or less in terms of a maximum value and 3 kxcexa9 or less in terms of an effective value under the dynamic condition.
As confirmed by the inventors, when such a rolling bearing that is suppressed in resistance between the inner ring and the outer ring under the certain condition is used, the radiation noise generated from the bearing can be suppressed to such a level that causes no problem from the standpoint of the self-imposed standard of VCCI and of the standard of FCC.
The reason why the dynamic condition for measuring the resistance is determined by the load and the relative difference in rotational velocity is as follows. In the case where a load applied on the rolling bearing is too large, an oil film is broken to make the three members, the inner ring, the rolling elements and the outer ring, each made of an electroconductive metal, in contact each other. In such a case, the electric conduction among the three members can be assured to such an extent that the radiation noise causes no problem. The resistance between the inner and outer rings of the rolling bearing having electroconductive grease filled therein tends to be increased when the relative difference in rotational velocity between the inner ring and the outer ring. In view of the circumstances, the dynamic condition for measuring the resistance is determined as a radial or thrust load acting between the inner ring and the outer ring of 10 N and a relative difference in rotational velocity between the inner ring and the outer ring of 150 rpm. Furthermore, because the radiation noise is liable to occur when the rotation number is increased, the electroconductive rolling bearing is preferably used at a rotation number of about 75 rpm or more, and more preferably 100 rpm or more. In the case where the rotation number is low, the conventional grounding method can be employed since noise is difficult to occur. When the electroconductive rolling bearings are used in two or more sites in the apparatus, the number of parts constituting the apparatus itself can be reduced, and miniaturization of the apparatus can be realized.